The present invention relates to a filter circuit of the capacitor-resistor type and, more particularly, to a switched capacitor filter circuit of the m-derived type, in which an equivalent resistance of a switched capacitor citcuit is used as a resistor element.
There is currently a strong demand for small, inexpensive IC filter circuits in industrial and civil electronics technologies, such as codec (coder and decoder), speech recognition, and speech synthesizing. The so-called leap frog method of filter formation is one which has long been known in this field. The filter circuits shown in FIGS. 1 and 2, arranged on the basis of the leap frog method, employ the equivalent resistance of the switched capacitor circuit. The third-order m-derived low pass filter circuit of FIG. 1 uses three switched capacitor circuits 1, 2, 3 as resistor elements, and three integrating circuits 4, 5, 6. The fifth-order m-derived low pass filter circuit of FIG. 2 uses six switched capacitor circuits 11 to 16 as resistor elements, and five integrated circuits 17 to 21.
The filter circuits shown in FIGS. 1 and 2 each need the number of integrated circuits equal to the order of filtering. Further, each integrating circuit is composed of an operational amplifier and a capacitor. Accordingly, the third-order m-derived filter of FIG. 1 needs three integrated circuits; while the fifth-order m-derived filter circuit of FIG. 2 needs five integrated circuits. This would indicate an increase in power dissipation. In addition to operational amplifiers, the filter circuit of FIG. 1 must use 11 capacitors and 20 switch elements; while the fifth-order m-derived filter circuit of FIG. 2 employs 11 capacitors and 22 switches. Requiring a number of circuit elements is problematic when the filter circuit is moduled into the integrated circuits, since it results in increasing the chips size.